Different upload directory based on post type in a theme

I have a lot of uploaded image files and image sizes. So it would be better to organize media files into folders based on post type. I have just read this tutorial, but as I can see it works with plugins. How to change this to use in a theme?
Thanks.

function custom_upload_directory( $args ) {
 
    $id = $_REQUEST['post_id'];
    $parent = get_post( $id )->post_parent;
 
    // Check the post-type of the current post
    if( "post-type" == get_post_type( $id ) || "post-type" == get_post_type( $parent ) ) {
        $args['path'] = plugin_dir_path(__FILE__) . "uploads";
        $args['url']  = plugin_dir_url(__FILE__) . "uploads";
        $args['basedir'] = plugin_dir_path(__FILE__) . "uploads";
        $args['baseurl'] = plugin_dir_url(__FILE__) . "uploads";
    }
    return $args;
}
add_filter( 'upload_dir', 'custom_upload_directory' );

If i understand your question right you want a function within your theme that adds directories for the current post_type? like: uploads/post_type_name. if so here is a function for that:

function wpse_16722_type_upload_dir( $args ) {

    // Get the current post_id
    $id = ( isset( $_REQUEST['post_id'] ) ? $_REQUEST['post_id'] : '' );

    if( $id ) {    
       // Set the new path depends on current post_type
       $newdir="https://wordpress.stackexchange.com/" . get_post_type( $id );

       $args['path']    = str_replace( $args['subdir'], '', $args['path'] ); //remove default subdir
       $args['url']     = str_replace( $args['subdir'], '', $args['url'] );      
       $args['subdir']  = $newdir;
       $args['path']   .= $newdir; 
       $args['url']    .= $newdir; 
    }
    return $args;
}
add_filter( 'upload_dir', 'wpse_16722_type_upload_dir' );